Integrated circuits generally have a system of metallized interconnects which couple the various devices fabricated in a semiconductor substrate. Typically, aluminum or some other metal is deposited and then patterned to form interconnect paths along the surface of the silicon substrate. In most processes, a dielectric or insulative layer is then deposited over this first metal layer, via openings are etched through the dielectric layer, and a second metallization layer is deposited. The second metal layer covers the dielectric layer and fills the via openings, thereby making electrical contact down to the first metal layer. The purpose of the dielectric layer is to act as an insulator between the first metal layer and the second metal layer interconnects.
Most often, the intermetal dielectric layer comprises a chemical vapor deposition (CVD) of silicon dioxide. This silicon dioxide layer covers the first metal interconnects conformably so that the upper surface of the silicon dioxide layer is characterized by a series of non-planar steps which correspond in height to the underlying first metal lines.
These step-heights in the upper surface of the interlayer dielectric have several undesirable features. First, a non-planar dielectric surface interferes with the optical resolution of subsequent photolithographic processing steps. This makes it extremely difficult to print high resolution lines. A second problem involves the step coverage of the second metal layer over the interlayer dielectric. If the step height is too large, there is a serious danger that open circuits will be formed in the second metal layer.
To combat these problems, various techniques have been developed in an attempt to better planarize the upper surface of the interlayer dielectric. One approach, known as chemical mechanical polishing, employs abrasive polishing to remove the protruding steps along the upper surface of the dielectric. According to this method, the silicon substrate is placed face down on a table covered with a pad which has been coated with an abrasive material, also known as a polishing compound or slurry. Both the wafer and the table are then rotated relative to each other to remove the protruding portions. This abrasive polishing process continues until the upper surface of the dielectric layer is largely flattened.
Current all-polish planarization schemes are severely limited because polish rates are strong functions of feature sizes. For any polishing pad, narrow down areas polish more slowly than wide down areas, which are slower than wide up areas, which are slower than narrow up areas. "Down" areas refer to recessed portions of the semiconductor structure, and "up" areas refer to raised portions of the semiconductor structure. Up areas are often formed by the metallization lines referred to above, leaving the unoccupied silicon substrate surface as the down area. "Wide" up areas often occur as a result of a dense array pattern of metallization, while "narrow" up areas occur as a result of isolated metallization lines. Achieving a final planarized surface becomes a balancing act between several different polish rates and initial thicknesses of these wide and narrow up and down features.
Current back end of line (BEOL) planarization processes use either a simple dielectric polish or add a hard top polish layer. Modelled results for one example of an all oxide polish are shown in Table 1. This data is for a specific process with a specific pad. For the all oxide polish, a balancing act between the polishing non-uniformities and the feature size polish rate dependencies exist. Wider down areas planarize much more slowly than narrower down areas. More material must be removed to achieve planarity. When the step height associated with the 0.1 mm down feature is reduced to 1390 Angstroms, the differences in step heights between 0.1 and 0.5 mm down features exceed 1000 Angstroms with 20,000 Angstroms of material removed, leaving 2300 Angstroms of topographical variation due to typical polishing non-uniformities. Clearly, this simple oxide polishing process is not effective for planarization for down area feature sizes greater than 0.1 mm.
TABLE 1 ______________________________________ Modelling results for a simple oxide polish (Suba 500) Step Height =A!.sup.4 Amt Down Area Distance =mm! 3- down Pol.sup.1 0.1 0.5 1.5 sigma.sup.2 range.sup.3 ______________________________________ 1000 7329 7571 7753 120 424 3000 6153 6783 7284 360 1131 6000 4732 5751 6632 720 1900 10000 3334 4615 5852 1200 2518 15000 2153 3505 5006 1800 2853 20000 1390 2662 4282 2400 2892 40000 241 886 2292 4800 2051 75000 11 129 767 9000 756 ______________________________________ .sup.1 Amount of material polished and/or removed from uparea .sup.2 3sigma polishing uniformity of 12% in amount of expected variation .sup.3 Variation in down area step heights from 1.5 mm to .1 mm .sup.4 Step height calculated from model for down area dimensions and amounts removed as noted
In regard to placing a hard upper layer on the polished material, this offers a significant improvement. This process does not control dishing over wide metal lines, however, as no polishstop is present in these areas. Therefore, this process does not allow for over polishing to eliminate the approximate 2500 Angstroms of incurred polishing nonuniformities, in addition to feature size dependencies discussed above.
In addition, simple polish planarization processes cannot deal with diverse metallization densities, such as where wide unpatterned areas cover half of the chip and dense patterns cover the other half. For these applications, such single polish planarization processes will fail because non-planarities accumulate from level to level leading to such problems as tungsten puddling and problems associated with long contact and via overetches. This results in difficulties in photolithography, film deposition over steps, and via etching.
Thus, in spite of attempts at obtaining globally planarized surfaces, a need still exists for a method of obtaining such planarized surfaces while overcoming the problems discussed above.